Monochromatic imaging with reduced colored toner consumption and ESD defects

ABSTRACT

An imaging device prints in color and monochromatically, such as black-only. It has a plurality of photoconductive drums, one each for black toner and at least one colored toner. The drums contact and rotate with an intermediate transfer member (ITM) to transfer images at a first transfer. During black-only imaging, the photoconductive drum for the black toner rotates with the ITM at process speed while the drum for the at least one colored toner rotates at less than process speed, often substantially less. The technique reduces consumption of colored toner during black-only imaging and minimizes charge build-up on the drum for the colored toner.

The present disclosure relates to electrophotographic imaging deviceshaving black-only and color imaging, such as printers, copying machines,multifunction devices, etc. It relates further to controlling theconsumption of colored toner and minimizing or eliminating electrostaticdischarge (ESD) defects when imaging monochromatically, such asblack-only.

BACKGROUND

Color imaging devices contain two or more cartridges. Each transfers adifferent color of toner to a media sheet as required to produce a fullcolor copy of a toner image. A common imaging device includes fourseparate color cartridges—cyan, yellow, magenta, and black. Imageformation for each of the four colors includes moving toner from areservoir to an imaging unit where toned images, black or color areformed on photoconductive (PC) drums prior to transfer to a media sheetor to an intermediate transfer member (ITM) for subsequent transfer to amedia sheet.

When black-only imaging, mechanisms exist to separate or retract the PCdrums for the colored toners from contacting the ITM so their respectivemotors can be stopped. As this stops the drums from rotating, colortoner is not consumed. The mechanisms, however, add complexity and cost.Alternatively, PC drums for the colored toners are not retracted fromcontacting the ITM, but allowed to skid. This, however, causes anelectrostatic charge to develop on the surfaces of the drums. To avoidESD imaging defects, the drums are rotated occasionally to fullyrecharge them for subsequent imaging. The rotation uses colored tonereven though no colored imaging takes place. The rotation also wears thedrums. This can lead to early replacement. A need exists to overcome theforegoing problems.

SUMMARY

An imaging device prints in color and black-only. It has a plurality ofphotoconductive drums, one each for black toner and at least one coloredtoner. The drums contact and rotate with an intermediate transfer member(ITM) to transfer images. During black-only imaging, the photoconductivedrum for the black toner rotates with the ITM at process speed while thedrum for the colored toner rotates at less than process speed. If theprocess speed ranges from 2500-3000 rpm, the drum for the colored tonerrotates at 1000 rpm or less, perhaps on the order of 15 rpm or less. Ifthe process speed ranges from 75-400 mm/sec, the drum for the coloredtoner rotates at 10 mm/sec or less, perhaps on the order of 1 mm/sec.The technique reduces consumption of colored toner during black-onlyimaging. It also minimizes charge build-up on the drums for coloredtoners, which reduces ESD defects. No longer are complex retractionfeatures needed to separate the ITM from the surface of the drums forthe colored toner during monochromatic imaging.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of an electrophotographic imaging deviceaccording to an example embodiment showing monochromatic imaging, suchas black-only.

DETAILED DESCRIPTION OF THE EMBODIMENTS

With reference to FIG. 1, there is shown an imaging device 10 havingblack-only and color imaging capability. The device receives at acontroller, C, an imaging request 12 for black-only or color imaging.The controller typifies an ASIC(s), circuit(s), microprocessor(s), orthe like. The request comes from external to the imaging device, such asfrom a computer, laptop, smart phone, etc. It can also come internally,such as from a copying request. In any, the controller converts therequest to appropriate signals for providing to a laser scan unit 16.The unit turns on and off a laser 18 according to pixels of the imagingrequest. A rotating mirror 18 and associated lenses, reflectors, etc.(not shown) focus a laser beam 22 onto one or more photoconductive drums30, as is familiar. The drums correspond to supplies of toner, such asblack (K) and one or more colored toners, such as cyan (Cy), magenta (M)and yellow (Y). A corona or charge roller 32 sets a charge on a surfaceof the drums 30 as the drums rotate. The laser beam 22 electrostaticallydischarges the drums to create a latent image. A developer roller 34introduces toner to the latent image and such is electrostaticallyattracted to create a toned image on a surface of the drums. A voltagedifferential between the surface of the drums 30 and transfer rolls 36transfers the toned image from the drums to a surface of an intermediatetransfer member (ITM) 40. For monochromatic images, a toned image isapplied to the ITM from a single photoconductive drum. For color images,toned images are applied from two or more photoconductive drums.

The ITM 40, being entrained about a drive roll 42 and one or moreidler/tension rolls 44, moves in a process direction with the surface ofthe drums. A sheet of media 50 advances from a tray 52 to a transferroll 54 where a second difference in voltage between the ITM and theroll causes the toned image to attract and transfer to the media 50. Afuser assembly 56 fixes the toned image to the media through applicationof heat and pressure. Users pick up the media from a bin 60 after itadvances out of the imaging device.

The controller coordinates the timing of the image transfers andtransportation of the media from tray to output bin. The controllerprovides signals or not to pluralities of motors to cause them to rotateor stop. Motors 70 drive the rotation of the drums 30. The motors mayalso drive more than one drum and/or the drive roll of the ITM belt, asnoted by the dashed line at 42 in an alternate embodiment. The speed ofrotation is dictated by the signal from the controller.

During monochromatic imaging, such as black-only, the controlleroperates the motors in a fashion to prevent ESD defects, but withoutneeding to retract any drums from contacting the ITM or unduly consumingcolored toner. Namely, the drum for the black toner rotates with the ITMat process speed for imaging, but the drums for the colored tonersrotate at less than process speed while remaining in contact with theITM. It has been found that rotating the drums for the colored toner atany speed less than process speed is sufficient. To characterize theprocess speed, it has been observed that the drum for the black tonertypically rotates in a range of 2500-3000 revolutions per minute (rpm)during black-only imaging or, its surface velocity, moves in a range of75-400 mm/sec depending upon diameter. In such situations, the rotationof the drum for any of the colored toners should be rotated at 1000 rpmor less, more particularly at 10-500 rpm or even 15 rpm or less. Thisamounts to a speed reduction of 60% or more compared to the processspeed. In surface velocity, rotation of the drum for any of the coloredtoners should be 10 mm/sec or less, perhaps on the order of about 1mm/sec or less. As any rotation of the drums for the colored tonersexpends toner, although no color imaging occurs on the media, the slowerthe rotation of the drums the better for avoiding consumption of coloredtoner during black-only imaging.

The foregoing description of several methods and example embodiments hasbeen presented for purposes of illustration. It is not intended to beexhaustive or to limit the claims. Modifications and variations to thedescription are possible in accordance with the foregoing. It isintended that the scope of the invention be defined by the claimsappended hereto.

The invention claimed is:
 1. In an imaging device having color andblack-only imaging occurring at a process speed, including a pluralityof photoconductive drums one each for black toner and at least onecolored toner and an intermediate transfer member, a method ofblack-only imaging, comprising: determining whether said imaging iscolor or black-only; if said black-only, rotating throughout theblack-only imaging the photoconductive drum for the black toner at saidprocess speed while rotating the photoconductive drum for the at leastone colored toner at substantially less than said process speed; andduring said black-only imaging, keeping in contact the intermediatetransfer member and the photoconductive drum for the at least onecolored toner, wherein the process speed in revolutions per minuteranges 2500-3000 rpm further including rotating the photoconductive drumfor the at least one colored toner during the black-only imaging at 15rpm or less.
 2. The method of claim 1, further including rotating thephotoconductive drum for the at least one colored toner and theintermediate transfer member in a same process direction.
 3. The methodof claim 1, further including providing a separate motor for saidrotating the photoconductive drums for the black toner and the at leastone colored toner.
 4. The imaging device of claim 1, further includingproviding a plurality of motors, wherein one of the plurality of motorsis configured to rotate at least one of the plurality of photoconductivedrums and also the intermediate transfer member.
 5. In an imaging devicehaving color and black-only imaging occurring at a process speed,including a plurality of photoconductive drums one each for black tonerand at least one colored toner and an intermediate transfer member, amethod of black-only imaging, comprising: determining whether saidimaging is color or black-only; if said black-only, rotating throughoutthe black-only imaging the photoconductive drum for the black toner atsaid process speed while rotating the photoconductive drum for the atleast one colored toner at substantially less than said process speed;and during said black-only imaging, keeping in contact the intermediatetransfer member and the photoconductive drum for the at least onecolored toner, wherein the process speed in distance per time ranges75-400 mm/sec further including rotating the photoconductive drum forthe at least one colored toner during the black-only imaging at 15 rpmor less.